on the difficulties of stratigraphic hiatuses mamet b...
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ON THE DIFFICULTIESOF STRATIGRAPHIC HIATUSES
INTERPRETATIONBernard Mamet & Alain Préat
University of Brussels
MAMET B & PREAT A (2003) Sur les difficultés d’interprétation des hiatu stratigraphiques
(exemple tiré de la transition dévono-carbonifère, Bassin de Dinant).
Geologica Belgica 6/1-2:49-65, 6figs
THE AVESNOIS ‘BASIN’vs
THE DINANT BASIN
• a case study?• with possible consequence(s)…
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Van Steenwinkel, 1988
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THE PROBLEM ??? A biostratigraphic hiatus at the D-C transition
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Conodont guides defining theD-C/b ARE NOT recorded inthe Dinant and Avesnois basins
studied interval
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1-2: Epinette+Etroeungt+Avesnelles+Pont d’Arcole+Grives
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STRATIGRAPHYOF AVESNOIS
and of BOCAHUTQUARRY
(Mansy et al.,1988)
studiedinterval
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SEDIMENTOLOGY‘bed/bed analysis’
• ± 350 samples (100m) AVESNES/HELPE• 120 samples (40m) ANSEREMME
7 MICROFACIESFAMILIES
1mm
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MICROFACIES
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mf1 mf2
‘bac
kgro
und’
mf2=
mf1+Td
250µ 1mm
distaltempestite
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mf3
mf3=
mf2+T
mf4
mf4=
mf3+Tp
proximaltempestite
250µ
100µ
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mf5 mf6
250µ
1mm
FairWeatherWaveBase
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mf7
250µ
100µ
1mm
semi-restrictedplatform
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EVOLUTION
SÉDI
ME N T.
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REG
TRANS
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AVESNES-SUR-HELPE
• thick and ‘continuous’ profile• RAMP + PLATFORM settings (SWB-FWWB)• relative sea level fluctuations• erosive event near the ‘D/C b’
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EVOLUTION
SÉDI
ME N T.
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ANSEREMME• thinner and ‘discontinuous’ profile• RAMP + PLATFORM settings (SWB-FWWB)• strong salinity fluctuations• important erosion near the ‘D/C b’
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MICROBRECCIA ANALYSIS
microbreccia do not contain ‘pre-Hastière’ sediments/rocks (Etroeungt-Avesnelles)
BUT CONTAIN ONLY
Hastière sediments/rocks(reworked littoral lagoonal environments)
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INTERPRETATION?
biostratigraphy sequence stratigraphy D/C boundary
HIATUSES
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CORRÉLATION
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SEQUENCE STRATIGRAPHYVan Steenwinkel, 1993
SB1 SB1
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CONCLUSIONorigin of the hiatuses?
1 steady subsidence?2 facies constancy?3 sea level fluctuations?…. ? nb paraconformity as a rule
1’ numerous parasequences2’salinity and energy fluctuations3’ they surely exist BUT no detailed correlations are possible (Van Steenwinkel
1993 and this work….)
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CONCLUSIONorigin of the hiatuses?
SO WHAT?
BLOCK-FAULTINGcan match all the observations
• non deposition? (pre-Hastière)• erosion of an unknown
lower Hastière part
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bear in mind that…*the profiles are in a same basin, near each other*the succession is ‘apparently’ the same*the microfacies, the microfaunal/floral assem- blages and the lithologies are also the same …
these similarities are fallaciousdespite similar eustatic fluctuations
HALF OF THE SERIES ARE MISSING
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HASTIERE LIMESTONEcm-dm microsequences [18m at Ansermme]
• (1) proximal crinoidal tempestites• (2) ‘false’ peloidal grainstones with
calcisphaerids, Bisphaera, brachiopods, crinoids[open lagoon]
• (3) ‘false’ grainstones with very abundant ostracods and Earlandia minima + a few
Quasiumbella [lagoon]• (4) packstones-’false’ grainstones with micritized grains and ‘cut in bits’ lumps
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HASTIERE LIMESTONE
the (micro)parasequences are not
bathymetric BUT record
strong salinity fluctuations
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HASTIERE LIMESTONE
facies (1) to (4) evaporitic diagenesis[calcite pseudomorphs, idiotopic dolomite…] UNSTABLE ENVIRONMENTS WITH STRONG SALINITY FLUCTUATIONS semi-open to restricted with reflux brines
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HASTIERE LIMESTONE(upper part)
• normal salinity with the photic zone of an openmarine environment
• return of plurilocular foraminifers, of girvanella…..
• same evolution at Avesnes AND Anseremme
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